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Full text of "The Cambridge Natural History"

iv                              CENTRES   OF   RADIATION                          103
very early types. Professor Osborn has argued that from this
early Eutherian stock there were two waves of progress, or,, as he
expresses It, <e two great centres of functional radiation." 1
The first was largely ineffective, the second has produced all
the Eutherian orders of to-day. These two divisions are termed
by him. " Mesoplacentalia" and " Cenoplaeentalia." The first
division embraces the Amblypoda and their descendants the
Coryphodonts and Dinocerata, many of the Condylarthra, the
bulk of the Creodonts and the Tillodonts. These creatures
persisted for a time, but died out in the Miocene. They were
mainly distinguished by the smallness of their brain; the great
specialisation of structure which they exhibit having left that
organ unaffected, and therefore tending in the long run to render
them unable to cope with changes in the inorganic and organic
world. The successful division of the primitive Eutheria com-
prises the groups which exist at the present day, and is not
connected directly with those small-brained Mesoplacentals; it
has apparently originated, however, from the least specialised of
their ancestors. Professor Osborn thinks, moreover, that the Lemurs
and the Insectivores are persistent descendants of the earlier
wave of Eutherian life. It appears in fact as if ISTature had
created the existing Ungulate, Unguiculate, and other types on a
defective plan, and, instead of mending them to suit more
modern requirements, had evolved an entirely new set of
similarly-organised types from some of the more ancient and
plastic forms remaining over. The Marsupials may be the only
group of the early wave remaining, and they have been able to
hold their own for the geological reason that Australia was
early cut off from communication with the rest of the world.
That they are disappearing seems to be shown by their gradual
diminution as we pass from Australia towards the continent of
Asia, through the islands of the Malay Archipelago. Com-
petition has here decimated them, as it may do in the remote
future in Australia.
It is often said, but with some looseness of statement, that
ancient quadrupeds are huger than their modern representatives.
This statement is partly true in fact, but largely wrong in
implication. For it suggests that—and the suggestion is often
expressed in "books that are not authoritative—huge animals
1  Trans. JSTew York Acaet. Set. aciii. 1894, p. 234.